1. Field of the Invention
The present invention relates to a steering gear shaft for a steering column of a motor vehicle and having at least one crash-deformable section formed as a corrugated tube at least a portion of a longitudinal extent of which that contains at least one of corrugation crests of the corrugation tube, is formed of several layers.
2. Description of the Prior Art
Steering gear shafts of steering columns with deformable sections formed as corrugated tubes are disclosed, e.g., in German Publication DE-196 31 214 A and European Publications EP-0 709 274A1, EP-0 872 401A2, and EP-0 701 070A1. Corrugated tubes, which form the crash-deformable sections, can be arranged in different sections of a steering column. In a steering column, a single corrugated tube can be used or several corrugated tubes can be arranged in different section of the steering column. The corrugated tubes, which function as crash-deformable sections, are compressed axially, on one side, and can be bent sideways under the action of non-axial forces, on the other side.
Different methods of manufacturing of corrugated tubes, which function as crash-deformable sections, are known. Such methods are disclosed, e.g., in European Publication EP-661 117A1, EP-298 832B1, EP-0 782 891 A1, and in German Publication DE-20 27 638A1. A corrugated tube for a steering column, which functions as a crash-deformable section, should meet specific requirements. On one hand, it should be easily deformable in case of a crash to be able to perform its function, with the deformation taking place in a predetermined manner. On the other hand, it should be able to withstand loads acting thereon during a normal operation over its entire service life. Therefore, the corrugated tube should meet very high requirements which should be determined by extensive tests. These test should examine, among others, the stability under the action of torques of predetermined values and under action of static, dynamic, and pulsatory loads. Further, the deformation behavior should be tested. In addition to meeting all of these different requirements, the corrugated tubes have to have as small dimensions as possible for space-saving reasons.
In order to provide for manufacture of corrugated tubes capable of meeting the requirements for crash-deformable section, European Publication EP-0 82 891A1 discloses a method of manufacturing of such corrugated tubes according to which the wall thickness in the region of corrugation trough is increased in comparison with the wall thickness in the region of the corrugation crests. In the region of the corrugation troughs greater loads act on the corrugated tube during transmission of a torque because it is in this region, the wall of the corrugated tube has a smallest distance from the central axis of the corrugated tube.
DE-20 27 638A1 discloses a method of manufacturing of a radially corrugated tube according to which a uniform wall thickness, different wall thickness and/or profile of a corrugated tube can be obtained from shaft to shaft or zonewise. Thereby, a corrugated tube, which functions as a crash-deformable section of a steering gear shaft and having different collapsible zones can be produced.
German Patent DE-32 24 308C2 and Swiss Patent No. 324,476 disclose a corrugated multi-layer metal bellows which is used as a portion of liquid or gas conduits, in particular, for sealingly connecting movable relative to each other, parts. The German and Swiss Patents relate to field remote from and not comparable with steering columns for motor vehicles. They solve different problem and relate to products which have to meet different requirements.
A multi-layer corrugated tube is also disclosed in German Publication DE-25 44 769A1. With the multi-layer, e.g., two-layer corrugated tube, the stability of the tube under the action of a torque acting thereon about its longitudinal axis does not change or changes very little in comparison with conventional, single-layer tube with the same wall thickness, whereas the deformation, in particular, a sideway buckling under the action of non-axial forces takes place when the acting non-axial forces are noticeably smaller. This result in improved crash characteristics in comparison with conventional steering columns during the transmission of high torque. At that, the outer diameter of the corrugated tube can be reduced.
An object of the invention is a steering gear shaft of the type described above and having at least one crash-deformable section formed as a corrugated tube having improved rigidity under the action of a torque acting about the tube longitudinal axis and adequate deformability in case of a crash.